Diesel engines, because of the way they operate, emit soot particles or very fine droplets of condensate or a conglomerate of the two (particulates) as well as typical harmful gasoline engine exhausts (i.e., HC and CO). These “particulates” (herein Diesel soot), are rich in condensed, polynuclear hydrocarbons, some of which may be carcinogenic.
As the awareness of the danger Diesel soot presents to health collides with the need for greater fuel efficiency that Diesel engines provide, regulations have been enacted curbing the amount of Diesel soot permitted to be emitted. To meet these challenges, soot filters have been used. When using such a filter, the filter must be periodically regenerated by burning off the soot. However, because the temperature where Diesel soot ignites is significantly higher than the normal operating temperature of a Diesel engine, a number of catalysts have been proposed to reduce the ignition temperature of the Diesel soot.
Generally, catalysts containing alkali or alkaline oxides have been used to substantially reduce the Diesel soot ignition temperature significantly as described, for example, in JP 2001-17449; WO 03/011437; US 2002/0132727 and US 2002/0197191. Unfortunately, these catalyst are volatile and destructive to the filters resulting in impractical short life times. In addition, these catalysts still have required substantial amounts of noble metal catalysts to reduce the HC and CO gases that are emitted along with the Diesel soot.
Other oxides such as rare earth oxides (e.g., U.S. Pat. No. 4,515,758; US 2002/0197191; US 2002/0044897; US 2003/0124037; WO 01/02083) and base metal oxides have also been used in conjunction with noble metal catalysts to attempt to lower the Diesel soot ignition temperature while also catalyzing the HC and CO emissions. Unfortunately, these catalysts have tended to required substantial amounts of expensive noble metal catalysts and/or rare earth oxides.
Therefore, it would be desirable to provide a catalyst for a Diesel particulate filter that avoids one or more problems of the prior art such as one of the aforementioned problems. In particular, it would be desirable to provide a catalyst that eliminates the amount of expensive rare earth oxide and noble metal catalysts that have been required in the prior art to oxidize soot, while still achieving long lifetimes.